1. Field of the Invention
The present invention relates to gas turbine engines, and more particularly, to an air/fuel mixer for a combustor. The type of gas turbine engine may be used in power plant applications.
2. Description of the Prior Art
Low NOx emissions from a turbine engine, of below 10 volume parts per million (ppmv), are becoming important criteria in the selection of turbine engines for power plant applications. The current technology for achieving low NOx emissions may involve a combination of a catalytic combustor with a fuel/air premixer. This technology is known as Dry-Low-Emissions (DLE) and offers a prospect for clean emissions combined with high engine efficiency. The technology relies on a higher air content in the fuel/air mixture.
However, flame stability decreases rapidly at these lean combustion conditions, and the combustor may be operating close to its blow-out limit. In addition, severe constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to stability problems, and richer than average pockets will lead to unacceptable high NOx emissions. The emission of carbon monoxide as a tracer for combustion efficiency will increase at leaner mixtures for a given combustor due to the exponential decrease in chemical reaction kinetics. Engine reliability and durability are of major concern at lean combustion conditions due to high pressure fluctuations enforced by flame instabilities in the combustor.
In a DLE system, fuel and air are premixed prior to injection into the combustor, without diluant additions, aligned for significantly lower combustion temperatures, therefore minimizing the amount of nitrogen oxide formation. However, two problems have been observed. The first is the stability or engine operability which provides decreasing combustion efficiency and, therefore, high carbon monoxide emissions. The stability of the combustion process rapidly decreases at lean conditions because of the exponential temperature dependence of chemical reactions. This can lead to flame-out and local combustion instabilities which change the dynamic behavior of the combustion process, and endangers the mechanical integrity of the entire gas turbine engine. At the same time, a substantial increase in carbon monoxide and unburned hydrocarbon (UHC) emissions is observed, and a loss in engine efficiency can be found under these circumstances.
It has been found that a key requirement of a successful DLE catalytic combustion system is the reaction of a perfectly mixed gaseous fuel and air mixture that is less than 1% variation in mixture fraction. Constraints on the system include less than a 1% pressure drop across the mixer. It is also important to develop a flow which will not generate flash-back or auto-ignition of the combustible fuel/air mixture.
It is an aim of the present invention to provide a diffusion gas mixture which is capable of providing a fuel/air mixture with less than 1% variation.
It is a further aim of the present invention to provide a gas mixer that has a pressure drop of below 1% while reducing the risk of auto-ignition and flash-back.
A construction in accordance with the present invention comprises a fuel/air premixer for a gas turbine combustor, wherein the premixer comprises an annular diffuser assembly placed in the airflow path, upstream of an inlet to the combustor, the diffuser assembly having an upstream section and a downstream section relative to the airflow path and including a plurality of concentric rings wherein a diffuser passageway is formed between each adjacent ring in a pair of rings; the passageway so formed including a converging cross-sectional portion at the upstream section of the ring assembly and a diverging cross-sectional portion at the downstream section of the ring assembly, and a gap is defined at the narrowest part of the passageway formed by the adjacent rings; and an assembly of concentric fuel manifold rings is provided upstream of the diffuser assembly whereby each manifold ring is located in axial alignment with a corresponding diffuser passageway whereby the air flows around the manifold rings and through the diffuser passageways, and fuel is delivered from the manifold rings into the airflow.
In a more specific embodiment of the present invention, the gap defined between the diffuser rings is determined by the formula M=ACd{square root over (2+L xcfx81xcex94P)}, where M is the mass flow, ACd is the effective flow area, xcfx81 is density of the air, and xcex94P is the pressure drop.
A feature resulting from the present invention is that the fuel is drawn into the airflow since the fuel is fed at very low pressures. Thus, the fuel is not being mixed into the airflow as in typical fuel nozzles where the fuel is fed under high pressure and relies on the fuel momentum for mixing, but instead it is the flow of the air around the manifold rings which draws the fuel and the air is mixed into the fuel. This method is very effective since more than 95% of the fluid is air and it is the air that is doing the work.